In a snowy abandoned town, Ellie has rigged up a stretcher and drag an injured Joel to safety. Hidden in the basement of a house, she tends to his wound. Joel wants her to take the rifle and go back to Tommy. Ellie tells him to shut the f**k up, wrapping him up in a coat and going in search of supplies.
Then we’re in a flashback. Ellie’s back at school in the Boston Quarantine Zone, running circuits round a gym while listening to her Walkman. An older girl – Bethany – steals her headphones, telling a furious Ellie to pick up the pace. At first Ellie tries to de-escalate the situation. But when Bethany taunts Ellie about her friend being gone, all bets are off. Ellie punches her.
She sits in the principal’s office with a black eye – meanwhile Bethany’s in the infirmary with 15 stitches. Captain Kwong tries to work out why Ellie’s been acting up. He recognizes Ellie’s potential, telling her she’s smart enough to become an officer and have a good life. He gives an inspiring speech and returns her Walkman.
That night Ellie reads Savage Starlight comics in her dorm. Then she stares at the empty bed opposite until lights out. As Ellie sleeps, an intruder climbs though the window. Ellie goes for her trusty switchblade. But it’s Riley. And Ellie is stunned – after three weeks of no news, she’d thought her friend was dead.
Riley asks who hurt Ellie, promising revenge. But Ellie won’t be distracted. She’s desperate to know where Riley went. And Riley admits that she joined the Fireflies. She invites Ellie to sneak out and have the best night of her life. At first Ellie’s reluctant – soon she’s meant to do drills where future soldiers learn how to kill Fireflies. But she can’t resist, and makes Riley turn round while she changes.
Ellie and Riley sneak out, narrowly avoiding a FEDRA patrol. They break into an apartment building and find a dead body on the top floor – a man who overdosed. They take his whisky right before the floor collapses, escaping out into the night, and pretend to like the taste.
On the rooftop Riley explains she’d been recruited after Marlene caught her out of bounds and was impressed by her skills. Ellie asks if it was easy giving everything up to join. But Riley doesn’t take the hint, talking about how much she hates FEDRA’s fascist regime and wants no part in it. Ellie echoes Kwong, arguing FEDRA holds the QZ together.
They hop across rooftops to an abandoned mall, continuing the argument. Ellie doesn’t want to go in because it was sealed off due to infected. But Riley scoffs, asking why the mall’s still open if that’s true.
Inside, Riley turns the power on. And Ellie’s enchanted by the magic of pre-outbreak life. The escalator blows her mind. She trips after goofing around, and Riley holds her close.
They stop to look at Victoria’s Secret, and Riley cracks a joke about imagining Ellie in lingerie to diffuse the tension. For a moment Ellie looks at her own reflection in the glass.
Then Riley gets Ellie to close her eyes, leading Ellie to a beautiful old carousel. They ride horses, laughing and swigging whisky. But the carousel grinds to a halt and the bubble is burst. The argument about how to make the world a better place continues, Ellie encouraging Riley to come back.
Riley reveals she’ll be 17 next month and got assigned to sewage detail; FEDRA never saw any potential in her. And Ellie wants to know why Riley never told her. But Riley panicked and ran until it was too late to return. She admits Ellie’s the one thing she misses. And Ellie cracks a smile. They take pictures in a photobooth, having fun posing together. Riley gifts Ellie the faded strip of pictures.
Next up is the arcade. They feed money into the machines and have a blast playing Mortal Kombat. But the noise travels through the mall, waking an infected wrapped in fungus. Oblivious, the girls play on until their bucket of coins is empty. They laugh, they scream, and almost kiss.
Ellie tries to head back to school, not wanting to get in trouble. But Riley lures her into the kitchen where she’s been camped with the promise of a gift: Ellie’s book of puns.
They swap some terrible jokes, giggling – until Ellie discovers a stash of pipe bombs. She realises Riley was posted in the mall, sent to guard weapons intended to kill FEDRA soldiers. Riley promises she’d never let the Fireflies hurt Ellie. But Ellie points out they’d never give her a choice and storms off.
Riley begs her to stay, confessing she’s being shipped to Atlanta; that Marlene refused when she asked if Ellie could join so they’d stay together. And this is her last night in Boston. Ellie says goodbye and stalks off, hurt and angry. She turns back at the last minute, running when she hears a scream. It’s a toy in the Halloween shop, which Riley saved for last, because she knew Ellie would love it most.
Ellie confronts Riley about leaving, and Riley admits she wants a family; that she matters to the Fireflies. Ellie points out that Riley mattered to her first. She checks Riley is sure about the mission and gives it her blessing.
Riley points out the night’s not over yet and plugs Ellie’s Walkman into the sound system. They try on masks and dance while Etta James sings I Got You Babe. Ellie takes off her mask and whispers, “don’t go.” And they kiss.
Just as Ellie and Riley begin planning a future together, an infected stumbles into the shop, drawn by the music. Riley shoots, but it just keeps coming. They run until the infected tackles Riley, knocking her out and sending the gun flying.
A screaming Ellie grabs her switchblade, stabbing the infected while trying not to get bit. They crash into a stand of costumes, waking Riley up.
She swings at the infected with a pipe, and Ellie stabs it in the head. The infected goes down. And Ellie is thrilled. Until Riley points out they’ve both been bitten.
Ellie rages against her fate, smashing up the shop. Then she sits down beside a tearful Riley, who takes Ellie’s hand and declares that whether they have two minutes or two days left, they’ll spend it together. Ellie and Riley hold each other tight.
Inspired by Riley’s determination to fight for every last bit of time together, present day Ellie ransacks the house, searching for anything that could help Joel. She pulls the kitchen apart and is rewarded with a needle and thread. Ellie races back downstairs, takes Joel’s hand, and stitches his wound.
